Applets are used to provide interactive features to web applications that cannot be provided by HTML alone. They can capture mouse input (like rotating 3D object) and also have controls like buttons or check boxes. In response to the user action an applet can change the provided graphic content. This makes applets well suitable for demonstration, visualization and teaching. There are online applet collections for studying various subjects, from differential equations[10] to heart physiology.[3] Applets are also used to create online game collections that allow players to compete against live opponents in real-time.

An applet can also be a text area only, providing, for instance, a cross platform command-line interface to some remote system.[11] If needed, an applet can leave the dedicated area and run as a separate window. However, applets have very little control over web page content outside the applet dedicated area, so they are less useful for improving the site appearance in general (while applets like news tickers[12] or WYSIWYG editors[13] are also known). Applets can also play media in formats that are not natively supported by the browser[14]

Java applets run at a speed that is comparable to (but generally slower than) other compiled languages such as C++, but many times faster than JavaScript.[15] In addition they can use 3D hardware acceleration that is available from Java. This makes applets well suited for non trivial, computation intensive visualizations.

HTML pages may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on the parameters that were passed. The first implementations involved downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation as slow loading components. However, since jars were introduced, an applet is usually delivered as a single file that has a size of the bigger image (hundreds of kilobytes to several megabytes).

Since Java's bytecode is platform independent, Java applets can be executed by browsers for many platforms, including Windows, Unix, Mac OS and Linux. It is also trivial to run a Java applet as an application with very little extra code. This has the advantage of running a Java applet in offline mode without the need for any Internet browser software and also directly from the development IDE.

Many Java developers, blogs and magazines are recommending that the Java Web Start technology be used in place of Applets.[16][17]

A Java Servlet is sometimes informally compared to be "like" a server-side applet, but it is different in its language, functions, and in each of the characteristics described here about applets.

Technical information

Java applets are executed in a sandbox by most web browsers, preventing them from accessing local data like clipboard or file system. The code of the applet is downloaded from a web server and the browser either embeds the applet into a web page or opens a new window showing the applet's user interface.

A Java applet extends the class Template:Javadoc:SE, or in the case of a Swing applet, Template:Javadoc:SE. The class must override methods from the applet class to set up a user interface inside itself (Applet is a descendant of Template:Javadoc:SE which is a descendant of Template:Javadoc:SE. As applet inherits from container, it has largely the same user interface possibilities as an ordinary Java application, including regions with user specific visualization.

The domain from where the applet executable has been downloaded is the only domain to that the usual (unsigned) applet is allowed to communicate. This domain can be different from the domain where the surrounding html document is hosted.

Embedding into web page

The applet can be displayed on the web page by making use of the deprecated appletHTML element,[18] or the recommended object element.[19] A non standard embed element can be used[20] with Mozilla family browsers. This specifies the applet's source and location. Object and embed tags can also download and install Java virtual machine (if required) or at least led to the plugin page. Applet and object tags also support loading of the serialized applets that start in some particular (rather than initial) state. Tags also specify the message that shows up in place of the applet if the browser cannot run it due any reason.

However despite object is officially a recommended tag, as of 2010, the support of object tag was not yet consistent among browsers and Sun kept recommending the older applet tag for deploying in multibrowser environment,[20] as it remained the only tag consistently supported by the most popular browsers. To support multiple browsers, object tag currently requires JavaScript (that recognizes browser and adjusts the tag), usage of additional browser specific tags or delivering adapted output from the server side.

Simple examples

A basic example using the java.applet package

The following example is made simple enough to illustrate the essential use of Java applets through its java.applet package. It also uses classes from the Java Abstract Window Toolkit (AWT) for producing actual output (in this case, the "Hello, world!" message).

importjava.applet.Applet;importjava.awt.*;// Applet code for the "Hello, world!" example.// This should be saved in a file named as "HelloWorld.java".publicclass HelloWorld extendsApplet{// This method is mandatory, but can be empty (i.e., have no actual code).publicvoid init(){}// This method is mandatory, but can be empty.publicvoid stop(){}// Print a message on the screen (x=20, y=10).publicvoid paint(Graphics g){
g.drawString("Hello, world!", 20,10);}}

For compilation, this code is saved on a plain-ASCII file with the same name as the class and .java extension, i.e. HelloWorld.java. The resulting HelloWorld.class applet should be installed on the web server and is invoked within an HTML page by using an <APPLET> or a <SCRIPT> tag. For example:

Displaying the HelloWorld_example.html page from a Web server, the result should look as this:

A Java applet example

Here it is: Hello, world!

To minimize download time, applets are usually delivered in a form of compressed zip archive (having jar extension). If all needed classes (only one in our case) are placed in compressed archive example.jar, the embedding code would look differently:

<P>Here it is: <APPLETcode="HelloWorld"WIDTH="200"HEIGHT="40"ARCHIVE="example.jar">
This is where HelloWorld.class runs.</APPLET></P>

Advantages

A Java applet can have any or all of the following advantages:

It is simple to make it work on Linux, Windows and Mac OS i.e. to make it cross platform. Applets are supported by most web browsers

The same applet can work on "all" installed versions of Java at the same time, rather than just the latest plug-in version only. However, if an applet requires a later version of the JRE the client will be forced to wait during the large download.

Most web browsers cache applets, so will be quick to load when returning to a web page. Applets also improve with use: after a first applet is run, the JVM is already running and starts quickly (JVM will need to restart each time the browser starts fresh).

It can move the work from the server to the client, making a web solution more scalable with the number of users/clients

If standalone program (like Google Earth) talks to the web server, that server normally needs to support also previous versions as the user may not keep it always updated. Differently, the browser updates the applet so there is no need to support the legacy versions. Only due configuration mistakes the applet may get stuck in the cache and have issues when new versions come out.

The applet naturally supports the changing user state like figure positions on the chessboard.

Developers can develop and debug an applet direct simply by creating a main routine (either in the applet's class or in a separate class) and call init() and start() on the applet, thus allowing for development in their favorite J2SE development environment. All one has to do after that is re-test the applet in the appletviewer program or a web browser to ensure it conforms to security restrictions.

An untrusted applet has no access to the local machine and can only access the server it came from. This makes such applet much safer to run than standalone executable that it could replace. However signed applet can have full access to the machine it is running on if the user agrees.

Disadvantages

A Java applet may have any of the following disadvantages:

It requires the Java plug-in which may not be available on some less popular web browsers or operating systems.

Some organizations only allow software installed by the administrators. As a result, some users can only view applets that are important enough to contact the administrator asking to install the Java plug-in.

As with any client side scripting, security restrictions may make difficult or even impossible for untrusted applet to achieve the desired goals.

If applet requires newer or specific JRE than available on the system, the user running it first time will need to wait for the large JRE download to complete.

Java automatic installation or update may fail if proxy is used to access the web. This makes applet with specific requirements impossible to run unless Java is manually updated. Java automatic updater that is part of Java installation also may be complex to configure if it must work through proxy.

Compatibility related lawsuits

Sun has made a considerable effort to ensure compatibility is maintained between Java versions as they evolve, enforcing Java portability by law if required.

The 1997 Sun - Microsoft lawsuit

The 1997 lawsuit [23] was filed after Microsoft modified its own Java Virtual Machine which shipped with Internet Explorer. Microsoft added about 50 methods and 50 fields[23] into the classes within the java.awt, java.lang, and java.io packages. Other modifications included removal of RMI capability and replacement of Java native interface from JNI to RNI, a different standard. RMI was removed because it only easily supports Java to Java communications and competes with Microsoft DCOM technology. Applets that relied on these changes or just inadvertently used them worked only within Microsoft's Java system. Sun sued for breach of trademark, as the point of Java was that there should be no proprietary extensions and that code should work everywhere. Microsoft agreed to pay Sun $20 million, and Sun agreed to grant Microsoft limited license to use Java without modifications only and for a limited time[24]

The 2002 Sun - Microsoft lawsuit

Microsoft continued to ship its own unmodified Java virtual machine. Over years it has become extremely outdated yet still default for Internet Explorer. In 2002 Sun filed an antitrust lawsuit, claiming that Microsoft's attempts at illegal monopolization have harmed the Java platform. Sun demanded Microsoft distribute Sun's current, binary implementation of Java technology as part of Windows, distribute it as a recommended update for older Microsoft desktop operating systems and stop the distribution of Microsoft's Virtual Machine (as its licensing time, agreed in the previous lawsuit, had expired).[24] Microsoft paid $700 million for pending antitrust issues, another $900 million for patent issues and a $350 million royalty fee to use Sun's software in the future.[25][26]

Applet security

There are two applet types with very different security model: signed applets and unsigned applets.[27]

Unsigned applet

Limitations for the unsigned applets are understood as "draconian":[28] they have no access to the local filesystem, web access limited to the applet download site, there are also many other important restrictions. For instance, they cannot access system properties, use their own class loader, call native code, execute external commands on a local system or redefine classes belonging to the certain packages. While they can run in standalone frame, such frame contains a header, indicating that this is an untrusted applet. Successful initial call of the forbidden method does not automatically create a security hole as access controller checks all stack of the calling code to be sure the call is not coming from improper location. Several specific security problems have been discovered and fixed since Java was first released, and some like [29] even persisted as long as till 2008 without anybody being aware. Some studies mention applets crashing browser or overusing CPU resources but these are classified as nuisances[30] and not as true security flaws. However unsigned applets may be involved into combined attack that exploit combination of multiple severe configuration errors in other parts of the system.[31] Unsigned applet can also be more dangerous to run directly on the server where it is hosted because while code base allows it to talk with the server, running inside it can bypass the firewall. An applet may also try DOS attack on the server where it is hosted but usually people who manage the web site also manage the applet, making this unreasonable. Communities may solve this problem via source code review or running applets on a dedicated domain.[32]

As of 1999 no real security breaches involving unsigned applets have ever been publicly reported, while these references are now dated.[30][33] Using an up-to-date Web browser is usually enough to be safe safe against the known attacks from unsigned applets.

Signed applet

Signed applet[34] contains a signature that the browser should verify through remotely running, independent certificate authority server. Producing this signature involves specialized tools and interaction with the authority server maintainers. Once the signature is verified and then the user of the current machine also approves, signed applet can get more rights, becoming equivalent to the ordinary standalone program. The rationale is that the author of the applet is now known and will be responsible for any deliberate damage. This approach allows to use applets for many tasks that are otherwise not possible by client side scripting. However this approach require more responsibility from the user, deciding whom he/she is trusting. The probable concerns include non-responding authority server (should the applet be allowed to run?), wrong evaluation of the signer identity when issuing certificates and known applet publishers still doing something that the user would not approve (as adding bookmark to the website). Hence signed applets that appeared from Java 1.1 may actually have more security concerns.

Java security problems are not fundamentally different from similar problems of any client side scripting platform. In particular, all issues related to the signed applets also apply to Active X.

Alternatives

Alternative technologies exist (for example, JavaScript, Curl, Flash, and Microsoft Silverlight) that satisfy some of the scope of what is possible with an applet. Of these, JavaScript is not always viewed as a competing replacement; JavaScript can coexist with applets in the same page, assist in launching applets (for instance, in separate frame or providing platform workarounds) and later be called from the applet code.[35]